Resonant Raman spectroscopy of organic semiconductors

Epitaxial films of a prototype organic semiconductor, 3,4,9,10-perylene-tet racarboxylic-dianhydride (PTCDA) are investigated with resonant Raman spect roscopy and density functional calculations. It is shown that the resonant Raman spectra are mainly determined by the vibrational properties of an iso lated molecule, and the pronounced subbands in the absorption spectra of di ssolved PTCDA molecules can be related to elongations of the Raman-active b reathing modes between the ground state and the relaxed excited state.